Technical Abstract:
Bronzing, copper top and sudden wilt symptoms in July and early August are usually caused by infections of Agrobacterium Biovar 1 isolates. The bacteria are seedborne and occur in all cultivars. The bacteria also occur in seeds of soybeans, peanuts, and pinto beans. The bacteria attack young seedlings at the natural wound sites caused by emergence of secondary roots. From 2 to 5 weeks after planting, many of the original secondary and tertiary roots are rotted and replaced by new "white" roots that originate from the same sites as the original roots. During this period the bacteria systemically enter the cotyledons and lower leaves causing bronzing, chlorosis, and epinasty symptoms. The bacterial concentrations in "white" roots increase two log orders (100-fold) from the 2-3-leaf stage to the 2-3-week-old boll stage. A compatible relationship is maintained during this time, and plants grow vigorously. As bolls approach maturity bronzing and defoliation of leaves, collapse and blackening of petiole and stem tissue, distortion of boll development, and extensive rotting of secondary and tertiary roots occur in some cultivars. Necrotic lesions or proliferated callus cells on main roots contain peak bacterial concentrations of about 100 million/gm fresh root. Late season symptoms are aggravated by early boll set and heavy fruit loads. Bacterial strains from different locations vary in their biochemical properties and toxin-producing abilities. Both "host plant resistance" to toxins and biological control with desirable Agrobacterium isolates appear to be feasible approaches to controlling the disease.